We have developed an apparatus that provides a high flux, low energy, monoenergetic positron beam to investigate various processes which occur when a positron beam impinges on a metal surface, including annihilation at the surface, trapping in vacancies, and the emission of both fast and thermally desorbed positronium. We report here the first angular correlation of annihilation gamma-rays measurements and positronium time of flight experiments at a material surface. We applied a simple free electron model which explains the general trend of the data but differences due to the surface specific properties and the deviation from a free electron metal are evident.

Traditionally, CD-SEM resolution has been measured using edge and spacing measurements, where images are subjectively examined and evaluated as to edge transition width between arbitrary amplitudes (e.g., 10% to 90%, 25% to 75%, etc.), or minimum measurable spacings between particles. Occasionally, methods from traditional optics, i.e., application of the Rayleigh criterion, have been quoted, but the more conservative results have caused users either to relax these traditional definitions or to abandon them altogether. More recently, as in light optics, Fourier methods, where frequency space results are used to define the resolution of the system, have been applied.In this paper, these methods are surveyed and applied to experimental data. In particular, Fourier transform approaches (including application of the David Joy SMART algorithm) are examined, and the difficulties in their application vis-A-vis separating signal from noise, maintaining repeatability, and eliminating system and processing artifacts addressed. A method based on the Rayleigh criterion for diffraction limited systems is proposed that minimizes these difficulties by processing out system effects and distancing the evaluation region from both the noise and system artifacts. This method has the particular advantage of not requiring operator intervention, ensuring consistent results from user to user.

Positronium velocity distributions using the time-of-flight technique have been determined for positronium emitted from clean, well-annealed, single-crystal samples of Al(111), Cu(100), Ni(100), and Au(100). The positronium energy distribution had a similar shape for all samples and was consistent with the hypothesis that positronium formation leaves behind a single electron hole in the conduction band of the metal. This was verified by explicit calculation for Ni, the first d-band metal to be studied by this technique.

The dependence of the positronium work function on temperature was investigated for five metals: Al, Au, Cu, Ni, and Pb. The slope of the dependence d\ensuremath{\Phi}/dT was found to be remarkably similar for the four materials, ranging from 5.3 to 9.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ eV/K, while the work function itself varied from -0.88 to -3.04 eV. The existence of the temperature dependence implies that the temperature variation in the electron and positron work functions is due to contributions from the chemical-potential term and not solely from the dipole-surface term as had been suggested.

Low-energy intense positron beams derived from pair production can be made at high-energy electron linacs and such beams are in operation or under installation at several linac facilities. Using a pulsed position beam made at a 100 MeV electron linac, we have measured the intensity and velocity distribution of positronium emitted from materials by measuring the time-of-flight of annihilating positronium. The time-of-flight data are augmented by positron lifetime and angular correlation measurements performed with the beam. Positronium spectra have been measured for a number of metallic samples. Several new observations have been made including details of the energy distribution of positronium emission formed by a thermalized positron and a conduction electron and the production of positronium from energetic positrons scattered out of the sample.

We report positronium time-of-flight and two-dimensional angular correlation of annihilation radiation measurements of the momentum distribution of positronium emitted from the surface of a lead single crystal. Contrary to earlier estimates of positive values, lead is found to have a negative work function (-0.73 eV) for positronium emission. A confirmation of the positive work function for positron emission is obtained and a value of 2.06 eV is deduced. Lead exhibits a work function positronium yield typical of the sum of positronium and positrons for metals with negative positron work functions. Evidence for thermal desorption of additional positronium was obtained, indicating the existence of a positron-positronium surface trapped state. The present data is consistent with a general description of work-function positron and positronium production resulting from a common interaction with the traversed material.

The production of positronium with kinetic energy greater than the positronium negative work function is clearly shown by time-of-flight spectroscopy of annihilating triplet positronium produced by the bombardment of low-energy positrons on metals. This higher-energy positronium channel is believed to be a consequence of positrons inelastically backscattered through the metal surface. At positron bombarding energies less than 100 eV, the positronium produced from backscattered positrons can be more intense than that produced by the diffusion of thermalized positrons to the surface.

Many data analysis treatments of positron experiments attempt to resolve two or more positron decay or exist channels which may be open simultaneously. Examples of the need to employ such treatments of the experimental results can be found in the resolution of the constituents of a defect ensemble, or in the analysis of the complex spectra which arise from the interaction of slow positrons at or near the surfaces of solids. Experimental one- and two-dimensional angular correlation of annihilation radiation experiments in Al single crystals have shown that two defect species (mono- and divacancies) can be resolved under suitable conditions. Recent experiments at LLNL indicate that there are a variety of complex exit channels open to positrons interacting at surfaces, and ultimely these decay channels must also be suitably resolved from one another. 6 refs., 4 figs.

A positron beam apparatus that produces a variable energy positron beam with sufficient intensity to perform new positron experiments in an ultrahigh vacuum environment has been installed at the Lawrence Livermore 100 MeV electron linac. We have installed two large area position sensitive gamma-ray detectors to measure angular correlations in two dimensions and a separate highly collimated detector to measure positronium energy distributions by time-of-flight velocity determination. Data from measurements on single crystals of Cu will be described.

We report the first two-dimensional momentum measurements of annihilating positron-electron pairs at a solid surface. Positrons of 18 keV or 740 eV impinged on a clean Cu(121) surface. The 740-eV spectrum was resolved into two components, one associated with energetic positronium emission displaced from zero momentum and a second centered on zero momentum with distinct asymmetry for the directions parallel and perpendicular to the surface. The 18-keV spectrum was dominated by positron Bloch-state annihilation.

Previous experiments with positrons from radionuclides have demonstrated that positron beams are a rich source of information about the surface condition of solids. We have now demonstrated the possibility of producing very intense beams at the Lawrence Livermore 100 MeV electron linac and installed an apparatus that produces a variable energy positron beam at energies between 500 eV and 20 keV with sufficient intensity to perform a variety of new positron experiments. The positron beam is pulsed with 10 ns to 3 μs duration at rates up to 1440 pulses per second, with as many as 106 positrons available per pulse. Experiments that require either pulsed or steady currents are possible in an ultrahigh vacuum environment. For the first time two-dimensional angular correlation spectra of the surface positron and positronium annihilation at a single crystal sample have been obtained for copper.

Recent advances in the use of positron annihilation to study defect ensembles in and on the surfaces of metals, are pointing the way towards studies where particular positron-electron annihilation modes may be identified and studied in the presence of one another. Although a great deal is understood about the annihilation of positrons in ostensibly defect free metals, much less is understood when the positron annihilates in complex defect systems such as liquid metals, amorphous solids, or at or near the vacuum-solid interface. In this paper the results of three experiments, all of which demonstrate means by which we can resolve various positron annihilation channels from one another, will be discussed.